The so-called thermite reaction traditionally involves the exothermic reduction of iron oxide with aluminum, in which the reaction produces molten iron with an aluminum oxide slag floating thereon, the reaction taking place either in a suitable mold so that the molten iron is fusion cast into a desired shape, or at a site where two metal parts are to be joined to produce a weld between such metal parts when the reaction is completed.
Although there are prior patents which involve the use of the thermite type reaction to produce borides, carbides, silicides and nitrides and the like, the product produced by the reaction is of at least two phases, one which is a layer of the boride, carbide, etc., and another which is a layer of the oxide of the reducing metal such as aluminum or magnesium. That is, the reducing metal oxide is present as a separate layer of slag, as in the classical thermite reaction. If special steps are taken to produce a composition which is a mixture of the boride, carbide, etc. and the reducing metal oxide, such composition is not a foamed product.
U.K. Pat. No. 1,497,025 teaches the production of cast refractory inorganic products by a thermite type reaction in which slag is formed and the product is a dense, sintered form. Thus, the teaching of this patent is directed to producing a composition which is not a mixture, homogeneous or otherwise, of all the reaction products, but of a composition which is a mixture of the reaction products less the oxide of the reducing metal and (to the extent possible) less the CO which is formed during the reaction. This patent is specifically directed to avoid "poorly sintered specimens" of the desired product and to avoid products which are characterized by "porosity and the presence of free carbon therein, which affects their strength". To this end, the patent teaches a method which is carried out at a centrifugal acceleration of from 100 to 1500 g and in a gaseous medium under pressure of 1 to 100 atm, using an inert gas such as argon. In this patent, the reaction mixture contains carbon and a reducing metal such as aluminum plus one or more metal oxides. The end product in each case is divided into two layers, a top layer of slag which is the reducing metal oxide and the bottom layer which is the desired material. Even if the constraints taught by this patent are not followed and porosity is present, it is not present in a composition which includes the reducing metal oxide.
Present techniques of producing refractory, monolithic shapes involve initial shape-forming steps such as hydraulic or isostatic pressing, slip-casting, extrusion, injection molding and the like prior to the firing step. Moreover, the firing step normally involves at least preheating the entire reaction mixture either to ignition temperature or to an elevated temperature at which local ignition and subsequent completion of the reaction occurs.